N(s) means what

In probability, the sample space ($S$) is the set of all possible outcomes of an experiment. $n(S)$ is simply the count of how many distinct outcomes are in that set.

For example, when rolling a standard six-sided die, the sample space is $S = \{1, 2, 3, 4, 5, 6\}$, and $n(S) = 6$.